It has recently been recognized that air turbulence in the vicinity of airports can pose a serious safety hazard. Such turbulence can be created in a number of ways. For example, the passage of an aircraft through a region of space is known to create helical turbulence behind the wing tips of the aircraft. Another turbulence source is the weather phenomenon known as a "microburst". In a microburst, a column of air descends rapidly towards the ground, and then fans out laterally. Both of these types of turbulence differ from the turbulence caused by thunderstorms and the like, in that they are entirely invisible to the naked eye. For this reason, such sources are often referred to as clear air turbulence.
Several approaches have been proposed and attempted for detecting air turbulence near airports, including those based upon radar, acoustic and optical technology. Many prior approaches rely upon the presence of rain drops, ice crystals, or heavy dust particles in the turbulent air, i.e., such techniques detect particles entrained in the moving air, rather than the moving air itself. Although techniques that rely upon the detection of particles moving in turbulent air can provide warning for many types of hazards, there is a need for a general purpose technique that also works well for clear air turbulence.